Electrolysis of alkaline metal chlorides and apparatus therefor



E. M LLER Feb. 18, 1941.

Filed June 24, 1937 mw 4 uw *bwl l \Wl YN. Y J 1 n 1 A. w

rich Hllen Stro- HCM Patented Feb. 18, 1941v PATENT OFFICE ELECTROLYSIS0F ALKALINE METAL CHLO- RIDES AND APPARATUS THEREFOB Erich Mller.Dresden, Germany, assignor, by

mesne assignments. to Th. Goldschmidt Corporation, New York, N. Y., acorporation of Delav v./liplilieation Jane 24, 1931, serial No. 150,021

In Germany April 13, 1937 2 Claims.

'lhis invention relates to electrolysis of alkaline metal chlorides andapparatus therefor; and it includes a process which comprises passing analkaline metal chloride solution through an electrolytic cell whileelectrolyzing the same, passing a stream of mercury through saidelectrolytic cell; the mercury serving as cathode and being converted toalkaline metal amalgam during the process; passing the resulting amalgamthrough an amalgam-decomposing cell having an inclined bottom. ofcatalytic material, formed in spaced parallel tracks, in contact with anamaigam-decomposing solution; the said mercury be-` ing passed into saidelectrolytic cell intermittently in pulsations of suificient magnitudeto produce a pulsating flow'through said cell and into the amalgamdecomposing cell where it is broken up into droplets which roll in saidtracks along the inclined bottom, thereby producing a' maximum surfaceexposure; collecting the mercury from the decomposing cell and returningit to the electrolytic cell. My invention further includes an apparatusfor conducting said process, said apparatus comprising an electrolyticcell and an amalgam-decomposing cell, means for introducing mercuryintermittently in a pulsating iiow into said electrolytic cell,means'for passing an alkaline metal chloride solution through said cell,means for passing the resulting amalgamated mercury from saidelectrolytic cell into said decomposing cell without substantialdisturbance of, said pulsating flow, means including asloping cellbottom formed in spaced parallel tracks ior passing the partiallyamalgamated mercury through said decomposing cell in the form ofdroplets and in contactwith a decomposing liquor, vand means forreturning the resulting mercury to the electrolytic cell; allasmorefully hereinafter setforth and as claimed.

Many different methods have been proposed in the prior art forelectrolyzing alkali metal chloridel solutions. In several of thesemethods use ismade of a owing mercurycathode. And a number of diierentelectrolytic cell designs have been proposed for this process. 'Ihemercury which acts as cathode usually ows over the floor of theelectrolytic cell while a. sodium chloride solution is passed over thismercury. 'I'his solution is electrolyzed as it passes through the cellbetween the mercury cathode and the anodes, usually of carbon. Chlorineis evolved at the anodes and is passed out of the cell and recovered.The mercury in passing through the cell is partially converted to sodiumamalgam and this is then decomposed by means of water or an aqueoussolution into mercury and causticlsoda in a separate decomposing cell.The hydrogen formed during the decomposition is collected separately. Acatalyst, such as graphite, is usually employed in the decomposing cellto assist in4 the decomposition of the amalgam. 'I'he mercury which isrecovered from the latter cell is then recycled to the electrolytic cellto be used in a repetition of the process. In the commercial operationof this lu process various difficulties have arisen.

When the conventional carbon anodes are employed in the electrolyticcell it has been found that these anodes tend to disintegrate slowly.This causes the production` of fine carbonparticles which, of course,tend to float on the sur- `face of the mercury or of the amalgam. Thesecarbon impurities, as Well as the iron amalgam impurities which areusually present, due to a slight reaction between the mercury and theiron of the containing vessels, tend to act catalytically causing. thedecomposition of the amalgam in the electrolytic cell. This leads to theformation of hydrogen gas which, of course, is liable to form explosivemixtures with the chlorine evolved in the electrolytic cell.

The difilculties caused by impurities in the mercury can be mitigated tosome extent by the provision of a plurality of transverse weirs acrossthe oor of the electrolytic cell over which the mercury is forced topass seriatim, the iloor of the cell being stepped downwardly from theentrance to the exit. The impurities are thus forced to pass over theweirs and a return is rendered impossible.

The same impurities cause diillculties in the amalgam-decomposing cell.In the conventional decomposing cell the alkali metal amalgam is slowlypassed across the iloor of the cell in countercurrent to a. flow ofwater, the floor of the cell being usually constructed of catalyticmaterial such as graphite. Grids of catalytic material are also providedwhich project above the surface of the mercury. It has been found thatthe passages between the grids become obstructed by the impurities inthe amalgam. Furthermore in this type of decomposing cell the floor ofthe cell is completely covered with amalgam which therefore also coversthe surface of any catalytic ma- 50 terial ofwhich the floor isconstructed. This surface is thereby rendered inactive. Even when a gridof catalytic material is employed, the decomposition of the amalgam inthe cell is slow. One cause for this is that the more dilute amal- 55gam is more dense than the more concentrated amalgam. 'I'he lattertherefore tends to float and to become'highly viscous on its surface,especially if it contains any substantial quantities 'of impurities. Theiiuidity of the surface of the amalgam is considerably less than that ofthe heavier and more dilute amalgam on the oorA of the cell. The amalgamtherefore tends to ow along the floor of the cell to the exit withoutcoming in contact with the decomposing liquor and therefore withoutdecomposing. The surface of the amalgam tends to become stagnant and aIdelayed decomposition as well as a lowered efficiency is an inevitableresult.V

I have found that the enumerated diiiculties which arise in thedecomposing cell can be eliminated in a surprisingly easy andcommercially feasible manner. 'Ihis is accomplished by slightly tiltingthe floor of the cell downwardly towards the exit and providinga'pulsating ilow of the amalgam to the cell. The amalgam then tends toform in droplets which are flattened more or less and which roll acrossthe oor of the cell thus continuously supplying fresh amalgam surfacesto the action of the decomposing liquor and also exposing the floor ofthe cell, thus rendering active its catalytic surface. 'I'he three-phaseline- ',contact of catalyst, amalgam and decomposing The rate ofdecomposition is increased still further by the provision of a catalyticgrid comprising spaced parallel bars running longitudinally along thefloor of the cell, these bars forming spaced tracks for the flow ofamalgam droplets. This prevents coalescence of the droplets and providesa uniform flow` of droplets across the floor of the cell. Furthermorethe total quantity of mercury-amalgam mixture in the decomposing cell isgreatly reduced and this reduces the quantity of mercury required in theprocess yas a whole. A pulsating ilow of mercury through theelectrolytic cell is employed in combination with a similar flow throughthe decomposing cell. It is usually possible to produce a positivepulsating flow at one point in the cycle, the pulsations carrying overfrom one cell to the other. If a posi tive pulsating flow is providedfor the electrolytic cell, by means of a pump for example, thesepulsations will cause the mercury to flow in pulsations throughout thelength of this cell and, if the amalgam leaving this cell is caused toflow into the decomposing cell in a manner preventing disturbance ofthese pulsations, the flow into the latter cell will also be in the formof pulsations, these pulsations being usually suicient to produce a flowof droplets across the floor of the decomposing cell in the mannerdescribed previously.

Itis possible, of course,v to produce independent and positivepulsations in the flow of the mercury through the decomposing cell.'Ihis can be accomplished by vibrating or rocking the decomposing cell,for example. If thedecomposing cell is vibrated these vibrations tend tobreak up the droplets of amalgam and to increase further the surfaceexposed to the action of the catalyst and decomposing liquor.

The pulsating flow in the electrolytic cell has the added advantage ofproducing a positive propulsion of the impurities towards the cell exit.

These floating impurities are lifted over any weirs provided in thiscell by the pulsations and may be collected in a well at the lower endand removed without disturbing the operation of the process.

It is also advantageous to provide a similar well at the lower end ofthe decomposing cell from which the impurities may be removed prior torepassing the mercury into the electrolytic cell.

It is advantageous but not essential to provide a counter current flowof water, as decomposing liquid, and of amalgam in the decomposing cell.The fresh water then comes into Ycontact with the most dilute amalgamwhile the most concentrated caustic alkali solution in the. cell iscontacted with the amalgam as it enters the cell. The stirring actionwhich is produced by the evolution of hydrogen in this cell can beminimized by the use of a low head of decomposing liquor in the cell.

My invention can be explained in greater detail by reference to theaccompanying drawing which shows an assembly of apparatus elementswithin the purview of my invention and useful in @the conduct of myprocess. In this showing,

gam-decomposing cell with related apparatus, l

while Fig. 2 is a vertical transverse section through theamalgam-decomposing cell, taken along the line 2-2 of Fig. l.

In the drawing like parts are represented by like reference numerals.The electrolytic cell is designated generally by the reference-numeralI, the amalgam-decomposing cell being similarly designated at 2. Thebottom of the electrolytic c ell is stepped, as shown, forming a seriesof trays 3B, 3b, 3c and 3d which are separated by the transverse wiers4I, 4b, 4, 4d and 4'3 over which the mercury 5 passes on its way throughthe cell.V

hollow cylinder 1 which is provided with a slot 8.v

The resulting pulsating iiow of mercury passes under the dam I2, overthe Weir IIa upon the tray 3a, then over the weir 4b and so on from trayto tray finally passing over Weir 4e through the opening I4 formed belowthe dam I5 and into the well I3 at the vopposite end of the cell. Thewell I3 is provided with 'a cover 20. The solution of alkali metalchloride enters the cell through the pipe I I and leaves at I'I. Thechlorine evolved is removed at I 8. The anodes, usually of carbon orgraphite, are shown at I9. course, forms the cathode. Electricalconnections to the anodes and to the mercury cathode are made as shownin the drawing. 'Ihe connection tothe mercury cathode may be madethrough the oor of the cell if this is made of metal.

The mercury is transformed into alkali metal amalgam in passing throughthe electrolytic cell and this is collected in the well I3. Variousimpurities collect, on top of this product and may be drawn 01T throughthe opening below the cover 20. The amalgam, substantially free fromim-` purities, is drawn olf by the tube 2| which connects with thebottom of the well I3. The amalv 2,232,128 gam is then passed into thewell 22 of the decom-A posingvessel or cell;

The decornposing cell 2 is provided with a sloping bottom 35 which isadvantageously made of catalytic material. such as graphite. A grid 23,usually o f'the same material. is supported on the bottom oi the cell.This grid is made of a series of spaced parallel strips, -as shown bestin Fig. 2, which serve to divide the mercury flowing along .the bottomof the cell into a plurality o1 parallel streams.

The pulsating ieed'to the electrolytic cell produces correspondingpulsations through the tube 2| to the decomposing cell. This pulsatingfeed causes the amalgam to flow over the bottom of the decomposing cellin the form of a series of droplets as indicated in Fig. 1, each impulsepropromoting a very rapid decomposition ofthe alkali metal amalgam.

The amalgam is decomposed in passing through the cell forming mercurywhich passes under the dam and falls into the Well 24 at the left handend of the decomposing cell. Any impurities which collect on the top ofthe mercury in well-24 canbe removed by skimming, for example, throughthe opening 26 at the top koi! the well 24, without disturbing theoperation of the process. The mercury is drawn oil. from the bottom ofthe well 24 and falls into a container 21. The pump 23 draws the mercuryfrom the container 2l, and delivers it to the vessel 6 from which pointit is again fed to the electrolytic cell,

to complete the cycle.

The decomposing cell is supplied with water or other decomposing liquorat 29 and the caustic solution which is formed during decomposition ofthe amalgam ilows out of the cell at 30. The hy drogen generated in thecell is removed at 3 I.

While, as stated previously, the pulsations produced by the cylinder 1,as the mercury is fed into the electrolytic cell, are generallysufficient to produce a corresponding pulsating ilow into thedecomposing cell, it isv frequently advantageous,`especially in ,thecase of large-scale operations, to provide for more positive pulsationsin the ow of the mercury along the floor of the decams a wave of thepartially amalgamated mercury flows over from the well 22 upon the iioorof the decomposing cell. 'I'his amalgamated mercury is broken up intodroplets by means of the grid 23 which droplets then roll over the oorVoi the cell as indicated in the drawing. Substantially the same resultcan be accomplished by the use of one ofthe conventional electricalvibrators which may be attached to the floor of the decomposing cell.The amplitude and the frequency of the vibrations produced in thismanner can be readily controlled in order to produce droplets of posingcell in such manner that the catalytic surface thereof is exposed to thedecomposing liquor. Subdivision of the mercury is advantageouslyproduced both longitudinally as well as transversely of the cell, bymeans of a grid, for example.

While I have described what I consider to be the best embodiments of myinvention, it is obvious that various modiiications can be madewithoutdeparting 4from the purview thereof. For example, either or boththe electrolytic cell and the decomposing cell can be vibrated or rockedto produce the desired pulsating ilow of mercury and amalgam. 'I'hepulsating iiow of amalgam to the decomposing cell can be supplied bymeans oi" a slotted piston similar to that shown at I in the drawing ifdesired. My process can be applied to the decomposition of any of thealkaline metal chlorides, including chlorides 'of the alkali metals andof the alkaline earth metals. The mercury or amalgam can be passed incounter current to or in parallel with the ilow of the aqueous liquor ineither or both of the cells. As a decomposition liquor, water, a causticalkali solution or any other aqueous solution can be employed. It is notessential that'the 4ilooi of the decomposing cell be made oi' catalyticmaterial since the use of a pulsating ow of mercury through this cell,in accordance with my invention, has advantages even when this iioor isof inert material. The impurities which collect in wells I3 and 24 maybe removed mechanically if desired either continuously or`intermittently. Other modicatlons which fall within the scope of thefollowing claims will be immediately evident to those skilled in thisart.

vWhat I claim is:

1. In the process of electrolyzing alkaline metal chloride solutionswherein such a solution is electrolyzed in a cell employing a flowingmercury cathode and the resulting amalgam is transferred by means of atube to an amalgam decomposing cell having a floor of catalytic materialformed into continuous spaced parallel tracks sloping downwardly fromthe entrance to the exit, the steps which comprisel introducing mercuryinto the electrolytic cell intermittently in pulsations of suilcientmagnitude to produce a pulsating ilow through said cell, through saidtube and into said amalgam decomposing cell at the upper edge of saidsloping floor, the ilow of amalgam alongthe upper edge oi' said slopingiioor dividing into a plurality oi streams, the pulsations in said iiowbeing sufcient in magnitude and the slope of said iioor being suiTlcientto cause said streams of amalgam to break into droplets at eachpulsation, whereby said droplets roll in said tracks to the'exit of theamalgam decomposing cell while being converted into mercury, collectingsaid mercury at the lower end of said sloping floor and returning it tothe electrolytic cell. y

2. In an apparatus for the electric decomposition of alkaline metalchloride solutions, an electrolytic cell provided with a ilowing mercurycathode fordecomposing such solutions in combination with an amalgamdecomposing cell having a floor of catalytic material formed-incontinuous spaced parallel tracks which'slope downwardly from thecatrame thereofA to the exit,

ymeansfor introducing mercury into said electroof 'said amalgamdecomposing cell; the pulsations' in said iiow being sumcient inmagnitude and the slope of said iloor being sufficient to subdivide theamalgam into droplets at each pulsation whereby said droplets flow insaid tracks across said s1oping iloor to the exit o! said amalgamdecomposing cell while being converted into mercury, and means iortransferring the resulting mercury to said electrolytic' cell. A

ERICH Mmm.

